The frosting of titanium white enamel [T.B. Yee, A.I. Andrews: J. Amer. Ceram. Soc. 39 (1956), p.188]is based on the crystallization of titanium dioxide from transparent glass matrix during stoving of the frit on the steel substrate. The desired optical properties such as brightness, color location [DIN 5033 and DIN 6174 or ASTM D 2244]and covering power [F.R. Meyer, Ber. Dtsch. Keram. Ges. 28 (1951), p. 205]are produced by the precipitation of particular TiO.sub.2 crystal phases in a particle size suitable for producing an optically cloudy effect.
Titanium white enamel has in the past been developed purely empirically. It is known that the B.sub.2 O.sub.3 /TiO.sub.2 ratio [V.V. Vargin, G.P. Smirnova, Steklo i Keram. (1962) 8, p. 35]and the alkali metal content [A.L. Friedberg, F.A. Petersen et al, J. Amer. Ceram. Soc. 30 (1947), p.261]influence the quantity of Ti02 precipitated while P.sub.2 O.sub.5 [R.D. Shannon, A.L. Friedberg, III Univ. Eng. Exp. Sta. Bull. 456 (1960)]promotes anatase recrystallization. Furthermore, high quality super-opaque enamels resistant to firing contain a few percent by weight of fluorine (A.L. Friedberg, F.A. Petersen et al, J. Amer. Ceram. Soc. 33 (1950), p. 17: F. Imoto, G. Yamaguchi et al, J. Ceram. Assoc. Jap. 61 (1953), p.97]in the melt composition. This element fulfills numerous functions. It assists opening up of the mixture of raw materials in the fusion process, facilitates melting of the frit during stoving by its lowering of the viscosity and stabilizing the precipitation of TiO.sub.2.
On the other hand, when fluorine is used as component of enamel frits, it causes ecological problems in the manufacture and processing of the frits. It gives rise to gaseous fluorine compounds, especially during melting of the mixture of raw materials and also during stoving of the frits. Removal of these gaseous compounds from the streams of exhaust gas issuing from the melting and stoving furnaces is difficult and expensive.
Attempts to develop titanium white enamel which is free from fluorine have hitherto only provided unsatisfactory results. The elimination of fluorine (A. Tomsia, C. Kozubek et al.,Mitt. VDEfa 24 (1976),p.71; D.F. Usakov, G.V. Kirilenko et al , Steklo i. Keram.(1982) 6,p.13) from titanium white enamel resulted in hard frits which not only necessitated higher stoving temperatures but also were distinctly inferior to conventional commercial enamel frits in their optical properties of opaqueness, such as covering power, and in their resistance to firing.
The problem therefore arose of producing enamel frits free from fluorine which would be comparable in their properties and in the production process to ordinary commercial enamel frits but would have the advantage of not causing any ecological problems in their production and processing and would not require an expensive process of working up the exhaust air.